The present invention relates to classifying systems for separating and classifying particles according to their size to produce separate discharges of fine product below a certain particle size and coarser product, and more particularly to product classifying systems of the centrifugal air type having means for achieving air suspension of the particles to be classified and vertical blade rotary rejector means for achieving separation and recovery of various small size material.
Heretofore, various centrifugal type air classifying devices have been provided to achieve separation of very small material, for example of the order of 10 to 12 micron material from particle material being supplied to the classifier, such as disclosed in earlier U.S. Pat. Nos. 3,384,238 and 3,615,009. While these prior art classifying machines are capable of recovering a small percentage of a 10-12 micron material from a mix of particle material being supplied to the classifier, it has been difficult to achieve the desired precision of the size limit or sharpness of cut in the very small size, ultra-fine size of cut, for example in the region of about a 5 micron topsize cut, and it has been very difficult to obtain a satisfactory percentage of fine recoverability when attempting to attain air classification of fine particles in the size range below 10 microns. My experience indicates that the recoverability of the position of fine particles in the 10 micron and below range by air classification compared to the percentage available is very low and attainment of reliable separation or topsize cuts in the range of about 5 microns has not been satisfactorily realized, mainly because of poor seal design.
The present invention relates to a centrifugal type air classifying system capable of achieving separation or classification at very small sizes, for example capable of separating fines with a 5 micron topsize cut and with considerably improved recoverability in the percentage of fine particles compared to the percentage available, by providing a main classifying chamber with a vertical blade rotary rejector, associated with a lower classifying or expansion chamber for return air, and a fine particle collector cyclone, incorporated in an air duct and fan loop including a fan to supply transport and particle suspension air for the classifying of the fines. The cut point or upper limit of particle size classified by the system is variable by increasing or decreasing the speed of the vertical blade rotary rejector. The material to be classified is delivered to the main or upper classifying chamber so that the material "floats or swims" to the classifying chamber in a fluffy or dispersed state, and a tapered blade configuration is provided for the vertical blade rotary rejector associated with a novel primary main seal and secondary safety seal between the positive pressure and negative pressure zones of the vertical blade rotary rejector. The blade configuration provides varying tip speed which is highest at the top of the vertical blade rotary rejector, causing more air to flow at the top of the main classifying chamber giving better dispersion, and allowing the bottom portion of the vertical blade rotary rejector to recover a higher percentage of the fine material entering the classifying chamber.
An object of the present invention is the provision of a novel centrifugal air type classifying system for effectively classifying materials of very small size, in the order of about a 5 micron topsize cut, which achieves improved percentage of fine particle recovery compared to the percentage of fines available through improved seal design and through the novel design of the vertical blade rotary rejector, and wherein the upper limit or cut point of the fine material to be separated may be varied by varying the speed of the vertical blade rotary rejector.
Other objects, advantages and capabilities of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings illustrating a preferred embodiment of the invention.